Best way to eliminate "common mode" noise from AC

One possible way would be to add common mode choke. It can be done by making few turns (as many as possible) of the power cord on the large ferrite (chose one with the highest permeability). When wires in the cable carry current in opposite direction (normal/differential mode) magnetic flux is induced in the ferrite in opposite directions resulting in cancellation lowering inductance almost to zero. Common mode interference induces currents won't cancel, being in the same direction, producing inductance in series with both wires. This scheme works well at high frequencies only since it is difficult to put many turns of power cable on the ferrite. It is possible to build common mode choke for lower frequencies. It is pretty much 1:1 metal core transformer with identical windings connected as a common mode choke (in series) and not as transformer (in parallel). As long as windings are identical inductance for differential signal will be zero. I suspect that many power conditioners might have common mode choke in addition to normal mode filter.

Yes, common-mode choke or a balanced isolation transformer, appropriately sized for the load. A number of the "line conditioner" manufacturers like APC and Belkin PureAV employ common mode chokes in their products. I have been using balanced isolation transformers with great success over the past 8+ years.

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A shielded isolation transformer can reduce common mode noise by as much as 12 dB. A balanced (60+/-) iso transformer can reduce common noise by as much as 20 dB.

Differential or transverse noise is more difficult to reduce but when it comes to power transformers, the idea is the opposite of audio transformers. You want limited bandwidth and a EI core is better than a toroid in this regard. Toroids are easier packaging and have less magnetic radiation.

Transformers are probably no more or no less effective than LP filters for transverse noise below 600 Hz, which includes several harmonics. Personally, more concerned with high frequency SMPS hash.